Reply

Abstract

We thank Dr. Batu for his comments on our paper (Eaton et al. 2007) on three-dimensional ground water flow in the Maquoketa Aquitard. We agree that the classic analytical methods for leaky aquifer analysis, developed by Neuman and Witherspoon (see reference citations given by Batu) and others, are still relevant. However, indurated and fractured rock aquitards like the one we discussed present additional complexities for flow and hydrogeologic characterization. The objectives of our paper were to highlight these complexities and present a conceptual model that provides a better understanding of flow in these systems, which are very different from the unlithified, less heterogeneous aquifer-aquitard systems for which the classic analytical solutions were developed. We welcome the opportunity to provide additional discussion of our work. Dr. Batu discusses mainly the vertical flow assumption in aquitards, analytical solutions, aspects of the MODFLOW numerical code, and steady-state vs. transient conditions of the hydraulic head data. Notably absent from his comment is one of the main emphases of our work, which is that layered sedimentary bedrock is typically fractured. Preferential flow occurs along bedding-plane fractures; the mechanical stratigraphy inhibits propagation of vertical fractures. Although we refer to the classic aquitard framework to understand the bulk properties of the Maquoketa Formation, we also emphasize the role of bedding-plane fractures in the three-dimensional flow system (Figures 2 and 10 in Eaton et al. [2007]) and present an innovative application of thin model layers in our MODFLOW model (Table 1 in Eaton et al. [2007]). The presence of bedding plane fractures is supported by ground water geochemical data (Figure 7 in Eaton et al. [2007]). In response to Dr. Batu’s specific points, the tendency toward aquitard vertical flow due to hydraulic conductivity contrasts in layered systems is elementary both for analytical and numerical solutions, but our work adds to this framework the effect of the bedding plane fractures we describe. Of course, we recognize that analytical solutions for leaky aquifers are valid for both upward and downward flow through the aquitard and that both upward and downward flow occurs under field conditions (Figure 5 in Eaton et al. [2007]). We again refer interested readers to Eaton and Bradbury (2003), in which there is additional discussion of monotonic trends in hydraulic head profiles under transient conditions. We also recognize that the analytical solutions for leaky aquifers assume horizontal flow in the aquifers. We point out again that the main focus of our work is to present a conceptual model for three-dimensional flow in an aquitard. The possibility of three-dimensional flow in aquitards was recently recognized and analyzed analytically by Sepulveda (2008). Finally, we point out the distinction between steady-state conditions, which have apparently not been reached in the Maquoketa Aquitard, and equilibrated long-term values for the purpose of field data collection. As we stated, the hydraulic head data we used in our analysis of the Maquoketa Aquitard were collected over a period of years, not months.